I have been in that situation and slowing...not braking hard...has let the passer get ahead with less danger. I have held my speed when the passing vehicle was really accelerating quickly.
But , I think you did ok

Any time on a 2 lane I run the PSL. If being passed I wait until he is even with my TV and slow slightly. Hate the thought of slamming on the brakes. If things get hot I'll have both hands on the wheel as it is likely most without the power to get by will not be able to push my TV off the road.

You said you were running the limit and above so what gives you impression that you have to help law violators. You are towing burning more fuel so you are paying more for the road(in taxes) as they are. If I maintain the speed limit they can line up like a funeral procession. But if I see someone trying to pass and not have the race car they think they had I will back off the throttle to let them around. Keep towing and you see this next they run 70 to pass you and then run 45 and 50 before a hill or where you cannot pass them. A lot are in a hurry to go nowhere. No one is hurt so count it good. If you are not seeing an idiot driving the chance might be you are the idiot or worse riding with them.

Sounds like you did the right thing. The only thing I can add is to make sure you know the laws where you are driving. Arizona recently passed a law saying that if you are holding up traffic, (5 or more vehicles?) regardless of whether you are at, below, or above the speed limit, you should pull over when safe and allow the traffic to pass you. Easier said than done, my dad once pulled over while towing a trailer and it took almost 1/2 an hour to get back on the road, but he still did the right thing.

I don't know and I may be wrong but I think slowing down enhances your chances of being caught up in the accident if one did happen. Think about if you were one of the other 2 vehicles in this situation. If you were the passer and realized you were not going to make the pass you would be on your brakes to fall back in line and if you can't because of other traffic, oh well, but the RV you were trying to pass is far ahead of you now and will be watching the mayhem in their rear view mirror. Now if you were driving the on coming vehicle you would see a car in your lane trying to pass a big RV and you would instinctly start to slow and instinctively move to the right, not to the left into the path of an RV. Your slowing will allow the RV who maintained speed to close and pass you more quickly and be clear of the possible accident you are about to be in. IMHO slowing down only increases your chances of being part of the possible wreck. You want the wreck to happen behind you, not beside or in front of you.

My experience with people who pass you when you're already going faster than the speed limit is NOT that they step on the brakes, but that they step on the gas, whether they have anything left or not. That's the mindset that had them passing you in the first place.

My experience with people who pass you when you're already going faster than the speed limit is NOT that they step on the brakes, but that they step on the gas, whether they have anything left or not. That's the mindset that had them passing you in the first place.

The OP asked what he should have done and then he said this in his post:

"so I hit the brakes pretty hard ... he should have been able to get around me and back on our side , but NO , when he saw the car coming the other way he hit his brakes and sent the other car scrambling to get out of the way"

Just what I would predict the passing driver to do if there was no way he was going to make the pass.

I know instinct want to make you slow down but I'm going to try and maintain my speed. I just think slowing down puts me in more jeopardy when trying to second guess someone who is trying to make an ill advised pass.

The OP asked what he should have done and then he said this in his post:

"so I hit the brakes pretty hard ... he should have been able to get around me and back on our side , but NO , when he saw the car coming the other way he hit his brakes and sent the other car scrambling to get out of the way"

Just what I would predict the passing driver to do if there was no way he was going to make the pass.

I know instinct want to make you slow down but I'm going to try and maintain my speed. I just think slowing down puts me in more jeopardy when trying to second guess someone who is trying to make an ill advised pass.

Yeah, this guy did. And what if the next guy speeds up? You just don't know what they will do. You've got a 50-50 chance.